It's patronising. Can you imagine using 'old mother' in the same way? Horrible. A mother is a mother, we don't need the little dig at age.
I was married in my early/mid 20s. I had finished university, had been with DH some time, loved on my own from the age of 19. I was shocked at some of the reactions, really shocked. The number of people who made comments about my age, tying myself down, how sad it was 
Yes I was younger than average, but I was not some sort of child bride- I was/am a fairly independent, confident, adult woman.
I find it strange how women in their 20s are infatilised when it comes to things like this.
I do think there is a real expectation/belief that women in their 20s are still 'girls', they must be thin and pretty and sexually available, they must squeal over shoes, and live for the weekend and Have Fun. It's not really about 'working on your career', not everyone has a 'career' and even those who do will get shafted when they have babies in their 30s.
Damned if you do, damned if you don't.